HA-Ahx-Ahx-Ub-VS (UbiQ-187) is a potent and specific inhibitor of deubiquitinating enzymes (DUBs) based on a C-terminal electrophilic vinyl sulfone (VS) group. UbiQ-187 can be used for activity profiling experiments and determining DUB inhibitor specificity. UbiQ-187 contains an N-terminal HA-tag (YPYDVPDYA), which is a peptide sequence derived from the influenza hemagglutinin protein and allows for the sensitive identification or purification by anti-HA antibodies and/or anti-HA-agarose. The HA tag is separated from the Ub N-terminus by two aminohexanoic acid (Ahx) linkers for efficient recognition of the tag. To eliminate Met1 oxidation, Met1 is replaced by norleucine, a well validated Met mimic.